1. Field of the Invention
The invention relates in general to a device for driving a light emitting diode string, and more particularly to a device for driving a light emitting diode string for applying in a backlight module.
2. Description of the Related Art
Conventionally, backlight modules are provided as the light sources for LCD panels, where the light can be produced by LEDs. LEDs are solid state semiconductor light sources, and have the following advantages: extra-long lifetime, low power, low operating voltage, low operating temperature, and quick response time. These are advantages that can not be matched by cold cathode fluoresce lamps (CCFL), and are the reasons to the wide use of LEDs in various illuminations and small scale backlight modules of cellular phones. It is becoming apparent that LEDs will gradually replace CCFLs in many applications.
FIG. 1 (Prior Art) shows circuit diagram of a conventional driving device for LEDs. The driving device 100 includes a DC voltage source 102, a DC chopper 104, a filtering device 106, and a LED string 108. The DC chopper 104 is used for controlling the electrical connection between DC voltage source 102 and LED string 108, and the LED string 108 is controlled to turn on or turn off accordingly, i.e. to light up or shut off. Since filtering circuit 106 has an inductance, the waveform of current I of LED string 108 forms triangular waves, as shown in FIG. 1B. As a result, the LED string 108 can not operate with a fixed conducting current. Even if a voltage-stabilizing capacitor is connected to the LED string in parallel to stabilize current I, the problem of long capacitor charging and discharging time prevents LED string 108 from able to be quickly turned on or off.